This invention relates to hydrogels of copolymers which are useful in the preparation of materials which are applied to body tissues, such as wound dressings.
Hydrogels are polymeric materials capable of retaining substantial quantities of absorbed water. Typically, cross-linked hydrophilic polymers can be prepared which absorb between half to five times their own weight to yield elastic solids. In the dry state, (in which they are sometimes described as Xerogels), the hydrophilic polymers are generally hard and rigid; in the hydrated state softer and elastic. The precise properties depend on the composition and the structure of the hydrogel and the amount of water absorbed. They are not soluble in water but swollen by it in a predictable manner.
Until now hydrogels have been made by crosslinking hydrophilic polymers. The crosslinks hold the molecules together and give it mechanical integrity, while the absorbed water cats as a plasticiser for the sections of hydrophilic polymer between the crosslinks, giving them mobility relative to each other. The hydrated hydrogel, therefore, behaves in a similar way to vulcanized natural rubber.
Cross-linking has generally been effected by incorporating a bi- or polyfunctional unsaturated monomer in the monomer mixture from which the hydrophilic copolymer is prepared. One of the most commonly used hydrophilic monomers which has been used for preparation of hydrogels is hydroxyethyl methacrylate (HEMA). Commercially available HEMA contains quantities of dimethylacrylates as impurities so that its polymers are generally cross-linked. Another method whch has been used to effect cross-linking of hydrophilic polymers is by irradiation of a solution of the polymer with gamma or X-rays or an electron beam.
Hydrophilic polymers have been used to prepare articles which come into contact with body tissue, such as contact lenses and surgical implants in which their biocompatability has been exploited.
European patent application No. 0107376 (Johnson & Johnson Products Inc.) discloses a wound dressing which is based on a transparent layer of a water-swollen cross-linked homopolymer of N-vinyl pyrrolidone (VP). The homopolymer of VP is first prepared and dissolved in water. A hydrogel layer is then formed by irradiating with gamma rays while contained within a polythene bag.
Chemical Abstracts, Vol 89, 1978, page 376 (Hidekatsu et al), also discloses a similar wound dressing which consists of a homopolymer of N-vinyl pyrrolidone or of vinyl alcohol which has been cross-linked by gamma irradiation of the homopolymer while retained between glass plates.
Cross-linked polymers have the disadvantages that they cannot be moulded or extruded from a melt or cast from a solution of the polymer and this limits the applications to which hydrophilic polymers can be put.